If I cook my 10 pound prime rib roast at 200°F 1 hour per pound will it still be med.rare?

For best results, always use a meat thermometer! All roasts have different amount of fat and bone, nobody can tell you when it will be cooked except a thermometer. Cheap investment! For medium rare 150°F is about right.

150°F is waaay beyond med. rare !!!

150 degrees is correct for rare to medium rare

Limecat is spot on... 150°F is way to overcooked... I would pull it out at 120-125°F... Let it rest to finish 125-130°F... The ends will be more well done and the center will be more rare.

Medium Rare Temperature: 130-135°F after resting.

Well I don't know how beef cooks in your part of the world, but I just made a 10 lb. standing rib roast today, perfectly medium rare to medium. I smoked it (yum!) in my kettle grill, but the same temperatures apply. Using my remote sensing thermometer I let the internal temperature rise to 125°F, then let the roast rest under foil and a towel until the internal temp rose to 135°F. I think next time I'll go 5 degrees less, and get a little more toward MR- to rare.

This is the first time I've used the remote thermometer and I'm excited about it. I don't have to open the grill (or oven) and lose the heat in order to measure the temp any more. Plus the device has a sensor for the temp inside the grill, so I know when to add more fuel.